We report studies of the surface reconstructions for both the Ga-face and the N-face of wurtzite GaN films grown using molecular beam epitaxy. N-face reconstructions are primarily adatom-on-adlayer structures which can be formed by room temperature submonolayer Ga deposition. These structures undergo reversible order-disorder phase transitions to 1X1 in the temperature range of 200-300 degrees C. Ga-face reconstructions, on the other hand, require annealing to high temperatures (600-700 degrees C) in order to form, and in most cases they are stable at those temperatures. The film polarity is found to be determined by the initial nucleation stage of the film growth.